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reverser unlocked 767

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reverser unlocked 767

Old 8th Jun 2004, 10:39
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reverser unlocked 767

practised the reverser unlocked procedure on the sim. recently. quite violent especially right after take off. slow reaction will most certainly end up in a crash which makes you wonder why the procedure , at least the initial actions, is not a memory item. is there a reason boeing did this.my initial thoughts were that by virtue of its design and construction, the reverser would not unlock at take off power ( also because have only heard of it happening in cruise).

would appreciate any incite on the issue.
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Old 8th Jun 2004, 12:25
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For details, you'll have to be more specific, especially as to Engine type...

NoD
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Old 8th Jun 2004, 12:27
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Sounds like a reverser deployed (as opposed to unlocked) experience. Unlocked scenarios usually just give indications, deployed give you the yaw that you just love
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Old 8th Jun 2004, 13:52
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Unwanted reverser deployment can occur in any phase of flight.

All it takes is two or more protection system failures and the servos open it up.

Cruise is not a big deal since you have lots of altitude and very effective airelons.

Early climb is bad news because you have little altitude and little effective airelons with the reverse eflux blocking them out.

Reverser locking devices are only as good as their quality checks before each flight and latent failures are your worse enemy.
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Old 8th Jun 2004, 15:01
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A thrust reverser deployment at cruise will be fatal, remember the LUDA 767, it tore the engine off. Thats why another gearbox lock was fiitted to the system.

Stu
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Old 8th Jun 2004, 15:12
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The auto-stow mechanism was improved and became a mandatory modification following the findings of the Lauda Air in-air reverser deployment.
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Old 8th Jun 2004, 17:46
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Many years ago in Cranbrook, BC, a 737 thrust reverser deployed on go-around after touchdown.

Missed the snow plough, but hit the ground hard

The accident report findings included:
  • it was possible to recover from the thrust reverser deployment
  • the thrust lever came back so hard that it broke the PF's thumb.
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Old 8th Jun 2004, 18:45
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Thumbs up Deja vu all over again

In the certification of an aircraft the FAA allows the manufacturer to validate the efficacy of a design by analysis or test. In the case of the 767-thrust reverser deployment Boeing felt that it would not pose too severe a problem for the pilot. However instead of actually causing an uncommanded deployment they felt it could be dangerous so they performed a computerized simulation to determine the alteration of the airflow in the area of the engine. The computer analysis determined that the pilot could maintain adequate control and that the problems resulting from the disrupted airflow would be minimal.

By analysis or test has come to bite a lot of airframe manufacturers in the ass. Any thing to save a few bucks.

The problem resulted from an inadequate FMECA which could have isolated the cause of the problem.


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Old 8th Jun 2004, 19:24
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A-320 reverser deployment

About a month ago an Indian Airlines A-320 had an actual inflight reverser deployment during initial climb. That engine was shut down and a/c returned safely to departure airport.
the investigation is still on,but it is reassuring to know that the "bus" is quite easily managable with an inflight reverser deployment situation.
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Old 9th Jun 2004, 00:29
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Thumbs up Yeah but what if.....?

To: leftseatview

?
but it is reassuring to know that the "bus" is quite easily managable with an inflight reverser deployment situation.
What if the uncommanded deployment happened during cruise flight? Would the pilot be able to secure the effected engine prior to the disturbance in the airflow under the wing effecting the wing aerodynamics

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Old 9th Jun 2004, 01:53
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A thrust reverser deployment at cruise will be fatal, remember the LUDA 767, it tore the engine off. Thats why another gearbox lock was fiitted to the system.
You have your facts wrong on both points.

Not at cruise and it certainly didn't tear the engine off until the aircraft started to break up in its dive into the ground.
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Old 9th Jun 2004, 13:15
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If my memory serves me correctly, from the Lauda investigation determined that survival from reverser deployment in cruise relied on immediately selecting the offending engine to idle.

At cruise power the flow breakaway and loss of lift on the wing caused an uncontrollable rolling moment.

Check your FCOM for procedure.
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Old 9th Jun 2004, 17:52
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Inboard inflight reverser deployment up to Vmo was approved in the DC8s. Even an asymetric reverser deployment at high speed was no drama.
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Old 10th Jun 2004, 09:30
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Lu,

FADEC will command zero thrust as soon as it detects a reverser deployment. Older engine types might have a mechanical system to reduce thrust in the event that a reverser deployed.

I suspect there will be a big difference in how such an event could be handled depending on what has caused the event to happen in the first place.

With a 'folding blocker door' reverser, if it has not been powered hydraulically or pneumatically to the open position then I suspect that the airflow through the duct will prevent it's full deployment and thus render it relatively easily handled. Lots of vibration and of couse a large assymetric effect, but overall manageable.

If, on the other hand, there is a serious fault in the selection and saftey interlocks such that the reverser is actually driven to the full reverse position (as per Lauda?) then the outcome will I believe be very different. A fully deployed big fan reverser at high speed would be a totally different ball game indeed!

Other types of reverser might have very different characteristics which would determine their response. I think that while GB's point is very valid, the aerodynamic effects of a big fan reverser and that of a JT3D would be very different.

Just my opinion!
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Old 10th Jun 2004, 12:32
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Inboard inflight reverser deployment up to Vmo was approved in the DC8s. Even an asymetric reverser deployment at high speed was no drama.
But definitely not approved for the Boeing model 767 as Lauda Air, 223 poor souls and their loved ones can attest.
Thrust reverser system certification by the FAA required that the airplane be capable of continued safe flight and landing under any possible position of the thrust reverser (FAR 25.933(a)(2)). However, wind tunnel tests and data used in the simulation of this accident demonstrated that aerodynamic effects of the reverser plume in-flight during engine run down to idle resulted in a 25 percent lift loss across the wing. Simulation of the event disclosed that the airplane was not capable of controlled flight unless full wheel and full rudder were applied within 4 to 6 seconds after the thrust reverser deployed.
Fifteen minutes and one second into the flight the co-pilot's voice was heard to exclaim, "ah reverser's deployed," accompanied by sound similar to airframe shuddering, sounds of metallic snaps and the pilot-in-command stating "here wait a minute." The cockpit voice recording ended twenty nine seconds later with multiple bangs thought to be structural breakup of the airplane.
See following link for sobering reflection:

http://www.rvs.uni-bielefeld.de/publ...aRPT.html#1.16
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Old 11th Jun 2004, 15:36
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If I understand the issue correctly, the problem is with the engine placement so close to the leading edge of the wing. When the reverser fully deploys at power, the airflow over the wing is much more seriously disturbed than the certification simulations showed that it would be. The disturbance causes a significant loss of lift on the affected side, creating a large roll moment in addition to the yaw moment. This is why immediate corrective action is required for recovery.

The higher the engine's power setting at onset, the greater the problem. The Lauda Air aircraft was climbing through 24,000 ft when the reverser deployed, thus the engines were at climb power settings.

Other aircraft aren't as effected by a reverser deployment because the airflow over the top of the wing is not as seriously disturbed as on the 767, so this is a special case for this aircraft.

Last edited by Flight Safety; 11th Jun 2004 at 16:46.
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Old 11th Jun 2004, 18:32
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Thumbs up Horses and barn doors. Which came first

To: FlexibleResponse

Thrust reverser system certification by the FAA required that the airplane be capable of continued safe flight and landing under any possible position of the thrust reverser (FAR 25.933(a)(2)). However, wind tunnel tests and data used in the simulation of this accident demonstrated that aerodynamic effects of the reverser plume in-flight during engine run down to idle resulted in a 25 percent lift loss across the wing. Simulation of the event disclosed that the airplane was not capable of controlled flight unless full wheel and full rudder were applied within 4 to 6 seconds after the thrust reverser deployed.
Isn't it a bit strange? The design guidelines in the FARs allow a manufacturer to demonstrate a level of safety on systems by performing a detailed test or if they so choose they can demonstrate the systems level of safety by performing an analysis. However when they simulated the accident using a wind tunnel and computer analysis it was determined that the pilot could lose control if a reverser deployed in flight.

Why didn't Boeing perform both the wind tunnel test and the computer analysis? Perchance Boeing was mislead by the manufacturer of the thrust reverser actuator that determined in his analysis that there were several levels of protection and that both would have to fail in order for the thrust reverser to deploy.

It is my understanding that the problem was caused by an internal leak that allowed system pressure to build up to the point that the shuttle valve or what ever valve was involved shifted position allowing system pressure to flow to the actuator. Incorporating a built in leak returning system leakage back to the return system could have prevented this. It was this tiny oversight that resulted in the deaths of the passengers and the crew. It should have been picked up on the FMECA. Better still the designer should have thought about the possibility of internal leakage.


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Old 11th Jun 2004, 23:42
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Boeing is not the sole aircraft manufacturer who demonstrated in-flight on a revenue flight, at climb conditions that their thrust reverser could deploy and cause the aircraft to be upset (turn completely over)

I'm afraid that too many readers are trying to oversimplyfy this to fit preconceived notions.

With the large fan engines it was judged too difficult to demonstrate freedom from unsafe condition at the most critical flight regime in a test flight where recovery (at climb conditions) could not be assured. Thus the manufacturer (Boeing, Airbus and Douglas) relied on a thorough fault tree analysis but with only two fail safe devices to prevent unwanted deployment.

In spite of such precautions the latency of hidden manufacturing or maintenance flaws in these protective devices eventually removed desired redundancy and the the deployment and upset to the affeceted flights from all three airplane designers occured (a little research just might reveal the ADs which had to lock out the affected reverser designs until they could add a third locking device.

I expect that the new designed aircraft will still not be able to demonstrate in-flight tolerance to a deployment, but they no doubt will have a third locking device and have to demonstrate reliability of all such devices.
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Old 12th Jun 2004, 09:39
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sorry took so long to reply. was stuck in lagos with the whole country on strike and no internet.

thank you all so much. i definately have more insight on the issue and from your responses i deduce that:

a) during testing it was not practical( read :safe) to exhaustively test all possible fault/ failure scenarios in critical phases of flight, and therefore the computer analysis was used.

b) by design the manufacturers have built in redundancies that would take care of or prevent any CONCEIVABLE failure/fault scenarios. ( an engineer told me that on our CF6B2-7F with pneumatic reversers the autostow feature has been "optimised" over the years and it will as far as he knows not allow unwanted deployment- i wasn't conviced)

and it is because of the above that it is not a memory c/list. I stand to be corrected.
Aluta Continua!!

thank you all for your time and effort once again.
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Old 12th Jun 2004, 16:21
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Regs

While we're discussing this accident, the applicable regulatory changes are worth noting here;

The current regulation reads as follows:

FAR 25.933 Reversing systems.
(a) For turbojet reversing systems—

(1) Each system intended for ground operation only must be designed so that during any reversal in flight the engine will produce no more than flight idle thrust. In addition, it must be shown by analysis or test, or both, that—

(i) Each operable reverser can be restored to the forward thrust position; and

(ii) The airplane is capable of continued safe flight and landing under any possible position of the thrust reverser.

(2) Each system intended for inflight use must be designed so that no unsafe condition will result during normal operation of the system, or from any failure (or reasonably likely combination of failures) of the reversing system, under any anticipated condition of operation of the airplane including ground operation. Failure of structural elements need not be considered if the probability of this kind of failure is extremely remote.

(3) Each system must have means to prevent the engine from producing more than idle thrust when the reversing system malfunctions, except that it may produce any greater forward thrust that is shown to allow directional control to be maintained, with aerodynamic means alone, under the most critical reversing condition expected in operation.

[Amdt. 25–72, 55 FR 29784, July 20, 1990]


The accident report states that the 767 complied with amendments 25-38 thru 25-45 when it was certified, so the applicable regulation at the time read:

FAR 25.933 states;

Reversing systems

(a) Each engine reversing system intended for ground operation only must be designed so that during any reversal in flight the engine will produce no more than flight idle thrust. In addition, it must be shown by analysis or test, or both, that

The reverser can be restored to the forward thrust position; or

The airplane is capable of continued safe flight and landing under any possible position of the thrust reverser.

(b) and (c) omitted

(d) Each turbojet reversing system must have means to prevent the engine from producing more than idle forward thrust when the reversing system malfunctions, except that it may produce any greater forward thrust that is shown to allow directional control to be maintained, with aerodynamic means alone, under the most critical reversing condition expected in operation.


The proposed amendment change in 1984 that led to the current regulation, changed the language of part 25.933, and the following explanation was given in that proposal for the language change:

Explanation: Unwanted, inflight deployments of thrust reversing systems have occurred on turbojet powered transport airplanes, sometimes with catastrophic results. To preclude further catastrophic occurrences of this nature, Sec 25.933 was amended to require showing that the reverser can be restored to the forward flight position or that the airplane is capable of continued safe flight and landing under any possible position of the thrust reverser. Shortly after Sec 25.933 was amended, it was recognized that the change failed to achieve the intended level of safety due to the use of the word "or." An unwanted, inflight deployment is generally accompanied by damage to the reversing system due to the dynamic nature of the deployment, particularly at high speed. Although it might be demonstrated that an undamaged reverser could be restored to the forward thrust position, there is no assurance that the reverser could be restored in an actual unwanted, inflight deployment due to the possibility of unpredictable damage. It is, therefore, essential that the airplane be capable of continued safe flight and landing under any possible position of the thrust reverser. Conversely, it is also essential that an operable reverser be restored to the forward thrust position whenever possible. In view of the above, the word "or" would be replaced with the word "and" to require showing that the reverser can be restored to the forward thrust position, if undamaged, and that the airplane is capable of continued safe flight and landing under any possible position of the thrust reverser. With this change, Sec 25.933 would be consistent with the original intent of this rule and with industry practice, and would, therefore, introduce no additional burden. Prior to the amendment of Sec 25.933, paragraph (a) clearly applied to all reversing systems intended for ground use only, including reversible pitch propellers. Due to an inadvertence, Sec 25.933(a) can now be erroneously interpreted to apply only to turbojet airplanes. The proposed change would clarify the applicability of this section. Certain other editorial changes would also be made for the sake of clarity. The use of the term "extremely improbable" in this context does not mean that a numerical analysis of failure rates is required.

There is a staggering difference between the use of the word or and the use of the word and in this regulation. Thank God they finally got it right, but unfortunately the 767 was certified to the older regulation that used the word or, which has left us with the airplane we now have. This aircraft is not as aerodynamically capable of remaining controllable after a reverser deployment as aircraft certified to the later standard.

To my knowledge, no aerodynamic fixes (such as repositioning the engine by changing the pylon, or changing the reverser to prevent upward direction of the reverser plume) have been incorporated into the 767, and the grandfathered regs it was certified to do not require them.

Last edited by Flight Safety; 12th Jun 2004 at 16:50.
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